1. Field of the Invention
The present invention relates to a dental implant for preparing tooth prosthesis. More specifically, this invention relates to a dental implant having the function for immediate loading after implant placement.
2. Description of the Prior Art
Modern dental implants are designed based upon the biological fact that a titanium-alloy implant and a bone integrate with each other very strongly, bearing a tensile strength more than 100 kg. This fact was found by Branemark of Sweden in 1969, and is called Osseointegration. Typical such osseointegrated implants comprise a tubular body portion called a fixture which is emblaced within a bore drilled in the bone.
As shown in FIG. 9, the dental implant 110 developed by Branemark is composed of a fixture 111, an abutment 112, a gold cylinder 113, an abutment screw 114, and a gold screw 115 (totally five pieces). The fixture 111 has a tublar body and is threaded externally, and when implanted in bone it is perfectly osseointegrated with alveolar bone after three to six months. The abutment 112 connects the fixture 111 to the gold cylinder 113 through soft tissue. The gold cylinder 113 connects the abutment 112 to a prosthesis (called an upper structure). The abutment screw 114 fixes the abutment 112 to the fixture 111, and the gold screw 115 fixes the upper structure to the abutment 112 through the gold cylinder 113.
In the method developed by Branemark for using the dental implant 110, it has been ruled out to load by connecting the prosthesis to the implant until three to six months after placing an implant in the alveolar bone under mucous membrane, in order to keep the implant at rest. This is called delayed loading protocol. It was reported that micromotion of the implant was produced by omitting the delayed loading period and when the amount of micromotion exceeded 100 .mu.m, some disorder was caused on the surface between the implant and the bone. On the other hand, in Branemark's method, even solid denture is contraindicated during the delayed loading period of three to six months after implant placement, as its protocol attaches importance for keeping the implant at rest during the period. Since this means to lengthen the necessary period for operation and to bring upon patient's pain, it has been regarded as a problem to be solved.
In order to overcome the drawbacks, it is indispensable to load immediately after implant placement without rest period. Hence, an effective countermeasure to prevent micromobility of the implant becomes necessary. Several types of implants using the mechanical locking means for securing the implant in place within the bore in the jawbone have been suggested. These types of implants have an expansion screw in the internal channel of the fixture, the lower half of the fixture being cut in to the end and divided into several legs, and when the expansion screw is rotated the legs are expanded radially and outwardly, causing the anchoring effect of the fixture in bone. The method to load immediately after implant placement is called immediate loading, and the design to secure the anchoring effect in bone by expanding the legs of the implant in immediate loading is called apical expansion design.
Referring to FIG. 10, the first dental implant 120 of apical expansion design issued in U.S. Pat. No. 2,721,387 to Ashuckian (1955) has spindle-like form imitating the socket of the extracted tooth. In the internal channel of the fixture 121 of the dental implant 120, an expansion screw 122 is inserted. By rotating the expansion screw 122, an expansion nut 123 is drawn upwards, thereby spreading apart two legs which are formed by dividing the lower half of the fixture 121. Thus, the fixture 121 is anchored in bone so as to prevent micromotion of the implant.
As shown in FIG. 11, the dental implant 130 of apical expansion design issued in U.S. Pat. No. 3,708,883 to Flander (1973) has the structure that the lower half of a cylindrical fixture 131 is divided into two legs and a frustoconical head 132 is attached on one end of an expansion screw 133 which is inserted into the internal channel of the fixture 131. By rotating a square nut 134 threaded into the other end of the expansion screw 133, the expansion screw 133 is drawn upwards together with the head 132 and the legs of the fixture 131 are expanded.
As shown in FIG. 12, the dental implant 140 of apical expansion design issued in U.S. Pat. No. 5,087,199 to Lazarof (1992) has the structure that the lower half of a cylindrical fixture 141 is divided into plural legs and an expansion screw 142 having a conical head 143 is inserted into the internal channel of the fixture 141. By rotating the expansion screw 142, the point of the conical head 143 goes down into the hollow 144 at the internal center of the lower portion of the fixture 141, and the legs of the fixture 141 are expanded.
Further, as shown in FIG. 13, the dental implant 150 of apical expansion design issued in U.S. Pat. No. 5,489,210 to Hanosh (1996) has the structure resembling closely to the dental implant 140 in that the lower half of a cylindrical fixture 151 is divided into plural legs and an expansion screw 152 having a conical head 153 is inserted into the internal channel of the fixture 151. By rotating the expansion screw 152, the point of the conical head 153 goes down into the hollow 154 at the internal center of the lower portion of the fixture 151, and the legs of the fixture 151 are expanded.
These types of implants of apical expansion design, as described above, have codimon mechanism that they have an expansion screw in the internal channel from the head to the lower end of the cylindrical fixture, the lower half of the fixture being cut in to the end and divided into plural legs, and when th e expansion screw is rotated are expanded radially and outwardly, causing anchoring effect in bone preventing micromotion of the implant.
However, these designs have the risk of micro-leakage of bacteria called microorganisms from the head of the implant exposed in patient's intraoral cavity to the bottom of the implant placed in bone, passing through the micro-gap between the male threads of the expansion screw and the female threads of the internal channel of the fixture. Therefore, the dental implants of these types of apical expansion design may fail unless precautions attention is paid to potential contamination in the apical region, which could cause serious damage to periimplant bone crucial to achieve osseointegration, so that they do not have any practical value.
An improved dental implant 160 of apical expansion design is described in U.S. Pat. No. 5,681,187 issued to Lazarof (1997). The dental implant 160, as shown in FIG. 14(A), has a cylindrical fixture 161, the lower half of which is divided into plural (four) blade-like legs. The fixture 161 has a circular ring-shaped internal shoulder 162 at the upper middle in the internal channel of the fixture 161. A screw head 163, the outer diameter of which is larger than the inner diameter of the internal shoulder 162, and a male screw portion 164 which is inserted through the internal shoulder 162 constitute an expansion screw 165.
When the expansion screw 165 is rotated, a frustoconically-shaped expansion nut 166 is drawn toward the middle of the implant with the result that, as shown in FIG. 14 (B), the legs 167 of the fixture 161 located within the socket of the extracted tooth are spread apart, thereby anchoring the dental implant 160 in bone so as to prevent micromotion.
By means of the structure of the dental implant 160 of apical expansion design described above, when the expansion screw 165 is rotated and the frustoconically-shaped expansion nut is drawn upwards, then the legs 167 of the fixture 161 are expanded and reactive forces to return the legs 167 to their original form work against active forces to expand the legs 167 outwardly. These reactive forces work so as to compress the screw head 163 of the expansion screw 165 to the internal shoulder 162 of the fixture 161, causing a close contact between the both. This is said to be effective for preventing microleakage of bacteria from the head portion of the dental implant 160 to the other end through the internal channel of the fixture 161.
In the actual product of the dental implant 160, an additional structure is added to reinforce the effect described above (Lazarof S, Hobo S, Nowzari H: "The immediate load implant system", Quintessence Pub. Co., 1988, Tokyo). As shown in FIG. 15, the expansion screw 165 has a 15.degree. reverse bevel 168 on the periphery of the apical side of its crew head 163, forming a circular incisal edge 169. When the expansion screw 165 is tightened to perform apical expansion, the incisal edge 169 of the reverse bevel 168 is compressed in close contact against the upper surface of the internal shoulder 162 of the fixture 161 and deformed by strong compressive force to thereby cause a cold welding effect, so that the passway for micro-leakage of bacteria is sealed. As a result, the effect to prevent microorganisms is reinforced.
However, according to recent clinical experiences, it has become clear that the effect of the dental implant 160 of apical expansion design, shown in FIG. 14, involves the following drawbacks.
As shown in FIG. 14(B), when the expansion screw 165 of the dental implant 160 is rotated and the expansion nut 166 is drawn upwards, the blade-shaped legs 167 formed by dividing the lower half of the fixture 161 are expanded and the points of the blade-shaped legs 167 cut in deep into peripheral bone and fixed. In that case, it cannot be avoided that mechanical stresses are concentrated on the points of the blade-shaped legs 167 and partly damage peripheral bone. Further, in the similar condition, when a strong torque is applied on the head 163 of the dental implant 160, twisting force is produced between the base and the point of each blade-shaped leg 167 of the fixture 161, and the legs 167 sometimes are broken at their bases. Still further, horizontal micromotion of the implant 160 is apt to be produced by a horizontal force applied on the head of the fixture 161 in case that the points of the blade-shaped legs 167 are fixed and the upper body of the fixture 161 is unstable.
On the other hand, during the functioning of the dental implant such as mastification, an occlusal force of 60 kg at maximum is applied downwards on the head 163 of the dental implant 160 exposed in the oral cavity. Since the sum of the cross sectional areas of the threaded shank 164 of the expansion screw 165 and the expansion nut 166 is far larger than that of the legs 167 of the fixture 161, when an occlusal force is applied, the expansion screw 165 and the expansion nut 166 remain at the almost initial position, while the legs 167 of the fixture 161 cut in deep into the bone and move downwards. As a result, the expansion screw 165 moves upwards relative to the fixture 161, making the base of the head 163 of the expansion screw 165 and the upper surface of the internal shoulder 162 of the fixture 161 set apart from each other and destroying the cold welding effect. Consequently, micro-leakage of bacteria through the slight gap between the both is invited. A clinical statistics showed that contamination by microorganisms of the dental implant 160 has been observed with a 3% occurrence rate.
Therefore, to overcome the drawbacks of the dental implant 160 described above, the following problems have to be resolved.
(1) Stresses concentrate on the points of the legs 167 when the legs 167 are expanded, and a twisting force is produced between both ends of each leg 167 when a torque is applied on the head of the fixture 161, while horizontal micromotion of the fixture 161 occurs when a horizontal force is applied on the head of the fixture 161. PA1 (2) Micro-leakage of bacteria occurs when an occlusal force is applied downwards on the head of the implant. PA1 1) Mechanical stresses do not concentrate on the points of the legs of the dental implants when the legs are expanded; and any twisting force is not produced between both ends of each leg when a torque is applied on the head of the dental implant, and further horizontal micromotions do not occur when a horizontal force is applied on the head of the dental implant. PA1 2) Micro-leakage of bacteria does not occur when an occlusal force is applied downwards on the head of the dental implant. PA1 a cylindrical body receivable within a bore provided in a jawbone of a patient and having an internal channel from an upper end to an under end of the cylindrical body, including an annular internal shoulder at an upper middle of the internal channel and a skirt portion on an under half of the cylindrical body, plural slits being cut in on the skirt portion so as to form plural blade-like portions in the axis direction of the cylindrical body, female threads being formed on an internal surface of upper portion of the cylindrical body, PA1 a nut body including a cylindrical portion inserted into a circular edge of the skirt portion of the cylindrical body and an inner cavity having female threads, PA1 a first bolt body including on its upper end a head whose outer diameter is larger than an inner diameter of the internal shoulder of the cylindrical body and including on its under side a shank whose outer diameter is smaller than the inner diameter of the internal shoulder of the cylindrical body, the shank having male threads externally and being engaged with the female threads of the nut body, PA1 a connecting body having an internal channel from an upper end to an under end of the connecting body, including an internal shoulder near the under end of the connecting body, the connecting body having an inner diameter smaller than an outer diameter of head of the cylindrical body and larger than an inner diameter of the internal channel of the cylindrical body, the head of the cylindrical body being slided into an under end of the internal channel of the connecting body, head of the connecting body being embodied with a prosthesis, and PA1 a second bolt body including a head whose diameter is larger than an inner diameter of the internal shoulder of the connecting body and smaller than an inner diameter of portion of the internal channel of the connecting body positioned above the internal shoulder of the connecting body, and including a shank having male threads externally and having an outer diameter which is smaller than the inner diameter of the internal shoulder of the connecting body, the male threads of the second bolt body being engaged with the female threads provided on the internal surface of upper portion of the cylindrical body, PA1 so that inserting the first bolt body into the internal channel of the cylindrical body through the internal shoulder of the cylindrical body and rotating the first bolt body while engaging the male threads of the shank of the first bolt body with the female threads of the nut body causes swelling of the skirt portion of the cylindrical body into a spindle-like shape and causes close contact between a lower surface of the head of the first bolt body and an upper surface of the internal shoulder of the cylindrical body, and PA1 so that inserting the second bolt body into the internal channel of the connecting body through the internal shoulder of the connecting body, rotating the second bolt body while engaging the male threads of the shank of the second bolt body with the female threads of the internal channel of the cylindrical body and producing close contact between an under surface of the head of the second bolt body and an upper surface of the internal shoulder of the connecting body causes embodiment of the cylindrical body with a prosthesis through the connecting body.